Biometric devices are increasingly common in computer or network security applications, financial applications, surveillance applications, and system access control applications. Biometric devices detect or image a unique physical or behavioral trait of a person, providing biometric data that can reliably identify the person. For example, a fingerprint includes a unique pattern of ridges and valleys that can be imaged by a fingerprint sensor. The image of the fingerprint, or the unique characteristics of the fingerprint, is compared to previously captured reference data, such as a reference fingerprint image. The identity of the person is obtained or verified when the newly captured fingerprint image matches the reference fingerprint image.
Devices that image fingerprints or other biometric data can be subject to noise and other errors. For example, noise can occur when a user holds an electronic device that includes a fingerprint sensor, or when the electronic device is connected to a charger that is plugged into a power source, such as a wall outlet. Noise from the charger and other sources can be introduced into the signals produced by the fingerprint sensor. Charger and other conducted-type noises can adversely affect a fingerprint sensor, particularly in that the noise can cause the capacitance measured by the sensor to be measured incorrectly. For example, the noise can result in a higher capacitance measurement, which produces an erroneous image of a fingerprint. Additionally, the noise can cause a loss of dynamic range of the fingerprint sensor.